1. Technical Field
The present invention relates to the monitoring and control of systems for delivering gaseous and liquid media, and in one particular application the monitoring and control of coolant in a fuel control system for gas turbine engines.
2. Description of the Related Art
There are numerous applications in which gaseous or liquid media is circulated or otherwise delivered through lines. For example, the coolant flow media could be circulated through a closed system of lines or other conduit to carry away heat from system components that generate heat or are required to operate in a high temperature environment. One such application is the fuel system of a gas turbine engine, such as the large, multi-stage turbines used in the power generation industry.
Gas turbine engines of this sort have a set of rotating turbine blades that compress air leading to a combustion chamber into which fuel is injected and ignited. Fuel is delivered through metering orifices to burners in the combustion chamber under pressure through a fuel line. Combustion of the fuel turns a downstream set of blades from which energy is extracted and which can also be used to drive the compressor blades. An array of combustion cans are arranged in the turbine, each with several burner nozzles that ignite the fuel at light-off and sustain combustion during operation. The combustion cans of the turbine are high pressure and temperature environments. It is typical for the environment surrounding the combustion cans to reach temperatures of 400° F., and for the combustion chamber temperature to near 2,000° F. The liquid fuel is consumed at a rate of about 20 gallons per minute at a high fuel pressure of about 1200 psig. This extreme environment is very hard on the fuel control components of the turbine fuel system, particularly for dual fuel turbines in which the during sustained gaseous burn, the liquid fuel system remains inoperable for long periods of time.
A principal concern is the formation of the coke, or the tarry deposits left after the distillate or volatile components of the fuel are driven off by heat, on the metering orifices and other working surfaces of the liquid fuel control components. Coke deposits arise primarily from the presence of residual fuel left in the fuel atomizer, burner nozzles, control valves, fuel manifolds, and other components subjected to the high heat of combustion. Residual liquid fuel left in the liquid fuel control components during gaseous operation will begin to coke at temperatures of about 250-280° F. in the presence of oxygen, which are well under the combustion temperature.
To reduce the effects of coking liquid fuel can be circulated through a heat exchanger to cool the temperature of the liquid fuel distillate to below the coking threshold temperature during operation of the turbine in gaseous fuel mode, see U.S. Pat. No. 6,729,135. However, this system requires a heat exchanger and either a separate fuel recirculation pump or increased duty on the main fuel pump. Moreover, because the recirculation lines carry liquid fuel, these lines, along with any recirculation control components, present yet another location for coking to occur when the recirculation system is not operating. To avoid this, during liquid fuel operation some of the liquid fuel must be made to bypass the combustor to flow through the recirculation system.
Dedicated cooling circuits can be used that avoid the aforementioned problems with using fuel as the coolant. However, this too can be problematic if not properly monitored and controlled. For one thing, if the coolant runs too hot, then adequate cooling may not be achieved such that components may undergo the coking problem discussed above. Also, if the coolant were to leak, otherwise flow at an insufficient rate, not only could coking occur, but in the event of a leak in which coolant is sprayed onto the turbine, “turbine rub” could result, a condition in which housing components of the turbine shrink or contract from cooling caused by the leak and interfere with the turbine blades. If left unchecked, turbine rub can cause significant damage to the large, rotating turbine blades and render the turbine inoperable. Further, if the coolant were to run too cold, for example if at an excessive flow rate or supply conditions were at an insufficient temperature, then the coolant could actually reduce the fuel temperature sufficiently to case “waxing”, a condition in which the fuel media, such as diesel fuel, begins to turn into a paraffin material. This condition disrupts the fuel delivery system and can similarly render the turbine inoperable. In addition to the waxing problem, excessively cold coolant can interfere with the proper operation of other components or sub-systems, for example, fogging of the optical pyrometer.